1. Field of the Invention
This invention relates to a semiconductor physical quantity sensor, and particularly to a semiconductor physical quantity sensor in which a movable electrode and a fixed electrode confronting to the movable electrode are formed by forming a trench in a semiconductor layer of a semiconductor substrate, and a method of manufacturing the same.
2. Related Art
This kind of a semiconductor physical quantity sensor has proposed in WO 93/22690 (or EP 591554A1), or JP-A-9-211022, for example. These documents disclose a capacitor detection type semiconductor acceleration sensor, which has a support portion formed by a elastic beam supported by an anchor portion fixed to a semiconductor substrate, a weight portion connected to the support portion, a movable portion consists of a comb-shape movable electrode integrally formed with the weight portion, and a comb-shape fixed electrode fixed to the semiconductor substrate so as to confront to the movable electrode.
In this kind of semiconductor acceleration sensor, a structure in which the movable portion and the fixed electrode are released is formed by using photolithography technique and by etching a semiconductor layer of the semiconductor substrate in general.
According to this semiconductor acceleration sensor, since one of two intervals between detection surface of the movable electrodes and detection surface of the fixed electrodes is increased and another interval is decreased, the sensor can detect an acceleration to be applied thereto by differentially detecting a change of capacitance due to the interval changes.
However, this kind of semiconductor sensor having such a structure, the movable portion may adhere to the semiconductor substrate or the fixed portion such as the fixed electrode. This adhesion may be caused by van der Waals bonding or hydrogen bonding generated at a contacted portion because this structure is made of a semiconductor. It is found that this adhesion is caused by the following factors.
The first factor is an electrostatic force generated during a handling the semiconductor sensor. In the case such the semiconductor sensor having the structure made by the photolithography is handed with a jig, the jig needs to contact with somewhere of the semiconductor sensor. When the jig contacts with the semiconductor sensor, a potential difference occurs between the movable portion and the fixed portion by being partly charged. As a result, the movable portion confronted the fixed portion is attracted to and contacted to the fixed portion by the electrostatic force, and is stuck there.
The second factor is a surface tension of an etchant or detergent used in manufacturing the semiconductor sensor. In general, after an etching (release etching) for releasing the movable portion from the fixed portion is performed, the semiconductor sensor is soaked in liquid such as pure water so as to substitute the etchant with the pure water, and then the semiconductor sensor is dried (evaporated) at a room temperature. In this step, the movable portion and the fixed portion are attracted each other by the surface tension of the liquid stagnated therebetween.
The adhesion due to the surface tension is likely to occur between a detection surface of the movable electrode and a detection to surface of the fixed electrode confronted with a small interval (detection interval), or between a weight portion and the fixed portion confronted to the weight portion in moving direction. The reason will be explained hereinafter.
Because it is effective to enlarge a detection capacitance to improve a sensibility of the capacitance type sensor, it is preferable to enlarge a confront area between the movable electrode and the fixed electrode or to reduce an interval therebetween. Furthermore, it is effective to lower a spring constant in a detection direction or a direction in which the interval between the movable electrode and the fixed electrode changes to enlarge a displacement due to the acceleration (physical quantity). That is, the spring constant in the detection direction of the movable electrode or weight portion is lowered.
It is known that an attraction force due to the surface tension of the liquid stagnated between the confronted surfaces depends on an area to be contacted with the liquid and the interval therebetween. On the contrary, the liquid is likely to finally stagnate at a narrow portion due to capillary phenomenon, as the evaporation of the liquid proceeds. Therefore, since the interval between the movable electrode and the fixed electrode is small, these electrode may be stuck each other, because the liquid is likely to stagnate therebetween, the area contacting with the liquid is large enough to cause a large surface tension, and the spring constant is small. Furthermore, since the weight portion also has a small spring constant, the weight portion is likely to adhere a confronted portion by the surface tension. In other words, the movable portion and the fixed portion are likely to adhere each other as the sensibility to be required is increased.
The third factor is an excessive shock (acceleration) externally applied to the semiconductor sensor when the sensor is dropped down during being handled, for instance. When the excessive shock is applied to the semiconductor sensor in a direction that movable portion moves, the movable electrode adheres to the fixed electrode at a portion where the detection interval is small. Here, the bonding described the above is generated at the stuck portion. Although the movable portion may be released from the fixed portion by applying a restore (spring) force, which is larger than the bonding force for restoring the structure, it is not preferable to do so because it may decrease the displacement due to the physical quantity and decrease the sensibility.